Conventionally, a Mach-Zehnder interferometer may be used in an optical modulator that modulates a light generated in a light source. In such an optical modulator, a signal electrode and a ground electrode are installed along parallel optical waveguides. In recent years, there is a variety of optical modulation method; therefore, many optical modulators are equipped with multiple Mach-Zehnder interferometers. In this case, multiple Mach-Zehnder interferometers are integrated on one chip, thereby making it possible to reduce the size of an optical modulator.
An optical modulator equipped with multiple Mach-Zehnder interferometers can generate a multi-level modulation signal when multiple different electrical signals have been input to the optical modulator. That is, different electrical signals are input from the outside to signal electrodes corresponding to the Mach-Zehnder interferometers, respectively, thereby enabling optical modulation by a multi-level modulation method such as DQPSK (Differential Quadrature Phase Shift Keying).
A connector may be installed in an input unit that receives an electrical signal to an optical modulator. However, if connectors are installed with respect to multiple electrical signals, the size of the optical modulator is increased, thereby increasing the mounting area. Accordingly, flexible printed circuits (FPC) having flexibility may be used in the input unit for electrical signal so as to miniaturize the device.
Specifically, multiple wiring patterns corresponding to multiple signal electrodes of an optical modulator are printed on an FPC, and an electrical signal output from a driver is input to the optical modulator through the wiring patterns printed on the FPC. Each wiring pattern is soldered, for example, to an electrode that outputs an electrical signal from the driver, thereby a driver-side end of the FPC is electrically connected to the driver. Furthermore, an end of the FPC on the side of the optical modulator is inserted into a notch part formed on the optical modulator, and is electrically connected to the optical modulator by soldering each wiring pattern, for example, to a coaxial terminal that projects downward from an upper surface of the notch part.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2012-182409
Patent Literature 2: Japanese Laid-open Patent Publication No. 2012-48121
However, the FPC is formed of flexible material; therefore, if multiple wiring patterns are printed on the FPC, and the width of the FPC is increased, the FPC may deform under its own weight. That is, as described above, one end of the FPC is inserted, for example, into the notch part and connected to the optical modulator; however, toward the other end of the FPC, near the center of the FPC in a width direction hangs down to a direction of a substrate under the weight of the FPC, and the other end of the FPC is arched, nearly U-shape in cross-section. Therefore, when the other end of the FPC is soldered to the electrode that outputs an electrical signal from the driver, wiring patterns located at both ends in the width direction and the center wiring pattern are connected to electrodes through solders which differ in thickness. Consequently, there is a problem that it is difficult to achieve impedance matching equally in the wiring patterns.
An electrical signal supplied from the driver to the optical modulator is a high-frequency signal; therefore, if there is an impedance mismatch, an S11 parameter, which indicates reflection in an input port of the FPC, deteriorates. Then, power gain is reduced by loss due to reflection, resulting in degradation of electrical signal waveform. Furthermore, the amplitude of an electrical signal input to the optical modulator is reduced, so, to compensate for the reduction in amplitude, output of the driver is increased, and power consumption is increased.